…you can see the forecast for Mead and Powell jumping up with the big snowpack, then dwindling as that potential big runoff turned into actual not-so-big-runoff. But it’s important to note that Lake Mead is still forecast to end 2018 226,000 acre feet above the projection made back in January, and Lake Powell is still projected to end 2018 up 1.8 million acre feet above the January projection. That’s a result of how the rules apportion water between Powell and Mead.

“Losses”, Daniel Kahneman and Amos Tversky wrote, “loom larger than gains.” This is one of the foundational principles of the field of behavioral economics. That’s what’s going on with the headlines. It felt like we had that water, and then we lost it.

But the final numbers are important. This year’s runoff has not been as big as we all hoped, but it was still enough to push projected reservoir levels up.

Several hundred billion gallons of water vanished from federal forecasts for Lake Mead over the past two months, but Bureau of Reclamation officials insist there’s no reason to panic.

In April, the bureau was predicting that the man-made lake east of Las Vegas would finish 2018 about 21 feet higher than it is today. Now the bureau is forecasting a 4-foot drop in the surface of the reservoir over the next 18 months — a difference of 25 feet.

But not to worry, said Rose Davis, spokeswoman for the Bureau of Reclamation in Boulder City. It’s too soon to say which scenario might turn out to be true…

The bureau released its latest forecast for the river and Lake Mead this week as part of its monthly operational study, which predicts the most likely reservoir conditions over the next two years.

The dramatic change in those projections over the past two months also illustrates how much of the reservoir’s fate is determined by policy, not Mother Nature.

The coordinated rise and fall of Lake Mead and Lake Powell upstream is governed by a complicated federal framework, implemented in 2007, that is designed to protect minimum water levels in the nation’s two largest man-made reservoirs through 2026.

In April, it looked as if the water would be high enough in Lake Powell and low enough in Lake Mead to trigger a much larger release than usual downstream to Mead in 2018. Forecasters now expect Lake Powell to fall just short of the so-called “equalization” level, resulting in a more modest water release.

Davis noted that even under the new, less promising projections, Lake Mead is still expected to be just high enough on Jan. 1, 2019, to avoid triggering the first federal shortage declaration on the Colorado River. Such a shortage would require both Arizona and Nevada to reduce their use of water from the river…

How much water is that?

Over the past two months, federal forecasters have significantly reduced their projections for the water level in Lake Mead. Instead of gaining about 21 feet between now and the end of 2018, forecasters now expect the lake to lose about 4 feet.

So how much water does that 25-foot difference represent?

Every foot in Lake Mead contains roughly 100,000 acre-feet of volume, so the recent adjustment by forecasters amounts to 2.5 million acre-feet of water. That’s equivalent to 814.6 billion gallons, which is enough water to supply the Las Vegas Valley for about a decade.